In recent years there has been increased demand for mechanical seal constructions which are of the double or tandem-type so as to permit continued operation of an apparatus even if one of the seals should fail. With this type construction, the stationary seal ring normally has rotatable inner and outer seal rings urged into sliding sealing engagement with opposite axial ends thereof. With this double seal arrangement, an intermediate chamber is formed within the seal construction between the inner and outer seals in surrounding relationship to the shaft, and a buffer or cooling fluid is supplied to this chamber so as to cool the seal faces. The cooling fluid is typically circulated through a closed system by an auxiliary pump disposed externally of the seal construction. With this arrangement, the two seals operate in tandem when the cooling fluid is at a very low pressure so that the inner seal forms the primary seal, but if the high pressure working fluid leaks into the chamber, then the outer seal prevents leakage. Alternately, a high pressure cooling fluid can be supplied to the chamber, which cooling fluid is at a pressure higher than that of the working fluid, whereby the arrangement functions as a double seal for preventing leakage of the working fluid.
While several manufacturers have commercialized double or tandem seal constructions of this latter type over the past several years, nevertheless one of the common drawbacks of these seal constructions is the excessive heat generated thereby, and hence the difficulty of circulating adequate quantities of coolant or buffer fluid therethrough. The necessary circulation has typically been achieved by means of an auxiliary pump disposed externally of the seal construction, although most users have objected to such system since the necessity of using an auxiliary pump increases the complexity and hence unreliability of the system.
In recognition of this problem, namely the adequate circulation of buffer or cooling fluid, one manufacturer has attempted to incorporate a pump within the mechanical seal construction. This has been accomplished by fixedly and concentrically securing an elongated pumping sleeve within the seal rotor in surrounding relationship to the rotatable shaft. This pumping sleeve has a spiral pumping groove formed in the inner periphery thereof. With this arrangement, however, the spiral pumping groove and its small cross section creates a severe restriction on the quantity of fluid which can be pumped or recirculated, and hence the effectiveness of this arrangement is seriously questionable. Further, the pumping sleeve occupies substantial space both axially and radially, whereby the overall seal construction becomes of greater size, and hence will not always fit within the stuffing box on some types of equipment.
Others have used an impeller-type pumping wheel secured to or formed on the seal rotor. This pumping wheel is disposed within the coolant chamber and hence occupies substantial axial and radial space. It also normally requires that the inlet and outlet ports be axially offset. The resulting seal construction is hence more costly, bulky and complex.
Still another attempt to provide increased circulation of coolant through the chamber of a tandem seal construction is illustrated by the arrangement of application Ser. No. 499,416, filed June 2, 1983, now U.S. Pat. No. 4,966,619, owned by the assignee of this invention. The arrangement of the aforesaid application incorporates a pump directly within the mechanical seal construction. This is accomplished by forming the rotatable shaft sleeve as a pumping rotor, such as providing the shaft with a plurality of slots therein in circumferentially spaced relationship. These slots communicate with the coolant chamber disposed between the inner and outer seals, and the coolant is supplied to this chamber through inlet and outlet openings which extend through the gland and are tangentially directed with respect to the pumping rotor. The rotation of the shaft, and of the sleeve carried thereby, hence effects pumping of the cooling fluid within the chamber, thereby effecting circulation of the fluid into and through the chamber and through the closed circuitry without having to rely on an auxiliary pump. While this arrangement has proven to work in a desirable fashion with some tandem seal constructions, nevertheless this arrangement cannot be utilized in all seal constructions inasmuch as it requires utilization of a substantially large shaft sleeve, and hence this arrangement will not always fit within the stuffing box on some equipment due to the radial space requirements.
Accordingly, the present invention relates to an improved mechanical seal construction, specifically a double or tandem-type construction, which overcomes many of the above disadvantages, and which is particularly desirable for use with equipment having minimal radial and axial clearance within the stuffing box. The seal construction of this invention does not require or utilize a rotating pumping rotor or equivalent, but rather provides a stationary ring, such as the gland ring, with a circulation inducing dam positioned within an annular chamber between the inner and outer seals, which dam preferably extends from the downstream side of the outlet to the upstream side of the inlet. The dam effectively closes off that portion of the chamber extending from the outlet to the inlet, whereby rotation of the shaft and surrounding sleeve effectively induces the coolant to flow into the chamber through the inlet and thence around the chamber to the outlet. The dam has end walls which effectively act as deflectors at both the inlet and outlet to assist the cooling fluid in its entry into and departure from the chamber. This arrangement has been experimentally observed to significantly increase the velocity, and hence the quantity, of cooling fluid flowing through the chamber so as to effect proper cooling of the inner and outer seals.
The improved seal construction of the present invention is bidirectional and preferably has radially oriented holes defining the inlet and outlet for the coolant, whereby the use of tangential holes can be avoided.
The circulation inducing dam, in the preferred embodiment of the invention, preferably has the ends thereof, as disposed adjacent the inlet and outlet openings, oriented at an angle of approximately 45.degree. with respect to the radial direction so as to efficiently deflect the cooling fluid in its directional transition both when flowing into the chamber at the inlet, and out of the chamber at the outlet.
The seal construction of this invention is hence desirable for minimal radial and/or axial space requirements, for dissipating seal-generated heat, for controlling temperature at the seal faces or in the seal cavity for high temperature pumping applications, and for use in situations involving shaft rotation in either direction without requiring any seal modifications.
Other objects and purposes of the invention will be apparent to persons familiar with seal constructions of this general type upon reading the following specification and inspecting the accompanying drawings.